The role of bacteria and/or bacterial products in psoratic arthritis has not been clearly delineated in contrast to the well-defined role played by bacteria or their products in the pathogenesis of spondyloarthritis. Salmonella, Shigella, and Yersinia infections are capable of inducing spondyloarthritis in genetically predisposed (B 27 positive) individuals. Recent work is suggestive of cross-reactivity ("molecular mimicry") between cell wall components of certain Klebsiella serotypes and a gene product closely associated with HLA-B27 in ankylosing spondylitis patients. There is mounting evidence suggesting that bacteria or their products may play a significant role in the pathogenesis of psoriasis and its related arthritis. First, is the known exacerbation following streptococcal infections, which is supported by the findings of increased ASO in certain subsets of patients. Second there are increased numbers of Staphylococcal and other aerobic organisms in the psoriatic plaques. It has also been shown that psoriatic patients have an altered immune response to streptococcal antigens and that this response may be associated with the major histocompatibility antigens. Additional evidence is the development of skin lesions reminiscent of psoriatic plaques upon injection into rabbit skin of cell-free filtrates of streptococcus pyogenes. The aim of this project is to investigate the role of Streptococcal and Staphylococcal organisms in psoriatic arthritis by means of in vitro lymphocyte function tests. Antibodies to these organisms will be sought by several techniques including an ELISA assay. The role of the monocytes in the described abnormalities of cell-mediated immunity of psoriatic patients will be carefully evaluated. In addition, since there is a well-established association between psoriatic arthritis and certain histocompatibility antigens (Bw38, B27, Cw6, DRw4 and Dw4), a series of experiments will be undertaken in order to establish cross-reactivity with streptococcal and staphylococcal organisms. The proposed research will study basic bacteriologic and immunogenetic aspects of psoriatic arthritis with regard to application in the diagnostic and therapeutic management of these patients.